1. Technical Field
The present invention relates to a structure, and associated method of formation, in which conductive bonding pads and associated circuit elements of varying height are located on the same substrate.
2. Related Art
A substrate, such as a chip carrier, typically has a top surface and a bottom surface wherein either surface, or both surfaces, has conductive bonding pads for electrically coupling the substrate to such devices as electronic assemblies (e.g., chips) and electronic carriers (e.g, circuit cards). A conductive bonding pad typically contains copper, but may alternatively contain, inter alia, nickel. Currently, all pads on a given substrate have the same thickness. A reduction in pad thickness generally conserves space on the substrate as a consequence of the outward sloping of pad sidewalls from the top of the pad to the bottom of the pad. The outward sloping is generated by the subtractive etching process used to form the pads. The outward, or trapezoidal, sloping causes the cross-sectional area of the pad at a pad-substrate interface to decrease with decreasing pad thickness for a given angular slope. The reduction of pad cross-sectional area at the pad-substrate interface allows the pad centers to be more closely spaced, resulting in an overall reduction of the substrate surface area required for implementing the design features of intended applications. The foregoing remarks regarding the use of thin pads to conserve space also apply to circuit lines coupled to the pads inasmuch as the circuit lines may likewise be formed by subtractive etching and consequently have sloping sidewalls. Indeed, a pad may be viewed as volumetric section of a circuit line to which a conductive interconnect, such as a wirebond interconnect or a solder ball, may be electrically and mechanically coupled. Thus, both thin pads and associated thin circuit lines improve space utilization. Pads (and associated circuitizations) may be categorized as to thickness. Such categories include thin pads, thick pads, and medium pads.
A thick pad (and associated circuitization), which typically has a thickness between about 17 microns and about 50 microns, can generally be used for coupling electrical devices and is especially useful for coupling a large solder ball, such as a solder ball of a ball grid array (BGA), to a substrate for subsequent attachment of the large solder ball to a circuit card.
A thin pad (and associated circuitization), which typically has a thickness between about 3 microns and about 10 microns, can be used for coupling an electronic assembly (e.g., a chip) to a substrate, by use of a wirebond interface (e.g., a gold wire). However, pads are typically made of copper and copper is unsuitable for making a direct attachment of a chip to a substrate by use of a gold wire. To mitigate this problem, the copper pad may be coated with a layer of nickel--gold, wherein a coating of nickel is formed on a top surface of the copper pad, and wherein a coating of gold is formed on the coating of nickel. With the nickel--gold layer over a copper pad, the chip may be wirebonded directly to the gold coating and this wirebond connection is generally reliable. A thin or thick copper pad, with an overlying nickel--gold layer, could also be used for attachment of a BGA solder ball. Note that a thin pad without an overlying nickel-gold layer generally cannot be used for direct attachment of a BGA solder ball, because the soldering process alloys some of the pad metal (e.g., copper) into the bulk of the solder material (e.g., lead/tin). Thus, if the pad is too thin, nearly all of the pad metal may alloy with the solder material, resulting in an unreliable mechanical and electrical connection.
A medium pad (and associated circuitization) has a thickness between about 10 microns and about 17 microns. A medium pad is particularly useful in flip-chip bonding of a chip to a substrate by use of a small solder ball. Such flip-chip bonding may be accomplished by the controlled collapse chip connection (C4) technique. The diameter of the small solder ball may be nearly an order of magnitude smaller than the diameter of a BGA solder ball (e.g., 2 to 3 mils for a small solder ball versus 25 to 30 mils for a BGA solder ball). The relatively smaller solder ball diameter allows the pad thickness for small solder ball attachment to be less than the pad thickness for BGA solder ball attachment, due to consideration of the alloying of pad metal with the solder material as discussed supra.
It is to be noted that a BGA solder ball can be directly soldered to nickel--gold coating over a thin copper pad, which conserves space. There is controversy, however, as to whether the solder-gold interface is susceptible to joint degradation. Thus, some designers and/or users may prefer to couple a BGA solder ball to a substrate by using a thick, uncoated copper pad than by using a nickel--gold coated thin copper pad. The decision of whether to couple a BGA solder ball to a substrate by using a thick copper pad or a thin nickel--gold coated copper pad is therefore discretionary and involves balancing the space-saving features of thin pads against reliability concerns associated with thin nickel--gold coated thin copper pads.
For applications requiring low-power input to a chip and low processing speed, it may be desirable to have thin circuitization throughout the substrate except where thick BGA pads are required. For applications requiring high-power input to a chip and high processing speed, it may be desirable to have thick circuitization throughout the substrate except where thin wirebond pads are required.
Currently, pads and associated circuit lines on a given substrate are of uniform thickness throughout the substrate. It would be desirable to have pads and associated circuit lines of differing thicknesses on the same substrate in order to benefit from the advantages associated with each pad thickness and circuit line thickness.